Methods of producing extreme flatness in heat treated stainless steel and the like



June 18, 1963 I D c. PERRY METHODS OF PRODUCING EXTREME FLATNESS IN HEAT TREATED STAINLESS STEEL AND THE LIKE Filed April 13, 1959 2 Sheets-Sheet 1 RH p ocssszua (Em. Pqacsss ('04 a IP04 L Ed Cam 3 6'0: 54 a 0 41007150 405 TE! 7': C'owal T/O/V/NG .Dsscwz E IIFOLLEfZEA EL I l 594w! I I Jase/us I IP01. 4 5e Lsr'z. (aw-lawn) SHEAR fapr/ama) 5/1557- PROCESS I JHEIK I INVENTOR. .0 64015490 P159 June 18, 1963 STAINLESS STEEL AND THE LIKE Filed April 13, 1959 2 Sheets-Sheet 2 (51.0 Bum (0 11.

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Descas (0P wow/4L) l/IP 5MP United States Patent Ofifice A 3 ,094,442 Patented June 18, 1963 3 094 4 42 METHODS OF PRODIlCIblG EXTREME FLATNESS IN HEAT TREATED STAINLESS STEEL AND THE LIKE D Cameron Perry, Middletown, Ohio, assignor to A rmco Steel Corporation, Middletown, Ohio, a corporation of Ohio Filed Apr. 13, 1959, Ser. No. 805,898 11 Claims. (Cl. 148-12) This invention relates to the manufacture of hard, flat, heat-treated stainless steels, and more particularly to techniques for producing extreme flatness in precipitation hardened stainless steels. From the outset it should be understood that while the invention is primarily directed to the treatment of precipitation hardened chromiumnickel steels, its utility is not so limited and the processing techniques herein disclosed may also be applied to other grades of stainless steel to produce sheets or plates for other applications wherein hardness and flatness are essential. Such additional applications include press plates, templates, saw blanks and the like.

A principal object of the invention is to produce extreme flatness in stainless steels while maintaining tensile properties equivalent to those obtained in products. hardened by standard heat treating procedures.

Flatness, as herein contemplated, may be defined in either of two ways:

(1) Flat within :.005 inch under 2 psi. vacuum, or

(2) Flat to less than 1% (height of buckle divided by the length of buckle).

Such flatness has hitherto been unobt-ainable in stainless and heat resisting steels whether hot rolled or cold rolled.

In this connection, there are presently no A.I.S.I. specifications for flatness tolerances in stainless and heat resistance steels in the three-quarter hard and full hard temper ranges. To date, flatness tolerances in stainless and heat resistant steels have been established only for the dead-soft, one quarter and one-half hard tempers of 2xx and 3xx series sheets wherein the permissible deviation is from /2" to 1% from a horizontal flat surface. Yet in accordance with the instant invention much closer flatness tolerances are obtained in extremely hard steels than were thought possible in steels of lesser hardness.

Accordingly, a further object of the instant invention is the production of flatness tolerances for stainless and heat resisting steels within the definitions enumerated above. For example, under the second enumerated definition, if a buckle were 20 inches long, a maximum height or deviation of .200 inch would be permitted.

In the percipitation hardening of chormium-nickel stainless steels, such as stainless steel of the l77 type, the precipitation agent is thought to be aluminum in some form since, when about 1% aluminum is added to the steel, the strength and hardness can be materially increased by a relatively low temperature final heat treatment following transformation from austenite to martensite. Two heat treating processes which are being currently employed to develop the strength and hardness properties of such steels are known in the industry as the RH (refrigeration hardening) and CH (cold rolling hardening) processes.

The first step of the RH process is an austenite conditionin-g treatment. The steel is thermally treated at a temperature sufficiently high to remove the effects of previous processing, yet not high enough to form stable austenite incapable of transformation to m-artensite when the steel is then cooled to any lower ternperatureeven sub-Zero temperatures. An austenite conditioning temperature range of 1600 to 1800 F. prevents transformation from austenite to martensite during the cool-down to room temperature. Yet, upon cooling to below room temperature, almost complete transformation can be effected. To insure complete and uniform transformation, the steel is preferably given a cold treatment at sub-zero temperature as soon as possible after conditioning. The refrigeration cycle consists of holding the work at temperatures of 0 to F. for a minimum time of one to two hours. Final hardening is then accomplished by a heat treatment at 850 to 1150" F. for at least about four minutes.

The CH process for heat treating is much simpler than the RH process, but it results in a less. desirable product from the standpoint of ductility, although it has extremely high strength and hardness. The starting material is a cold rolled coil which preferably has been cold reduced about 60 percent. While greater or lesser reductions may be employed, variations in strength will be encountered. The preferred reduction is higher than the normal cold reduction given stainless steel and is necessary in order to obtain substantial transformation of the steel from the austenitic to the martensitic condition. Although the material is extremely hard afiter this rolling, it can be further hardened by a heat treatment in the 850 to 1150 F. range. No thermal austenitic conditioning or refrigeration is employed in the CH process.

While both the RH and CH processes result in products which are extremely hard and have high strength characteristics, the products are not flat within the tolerances required for many present day applications; and by reason of their extreme hardness, conventional flattening techniques have proven inelfective.

In accordance with the instant invention and to develop precipitation hardened steels having properties comparable to those developed by the processing techniques just described, While at the same time developing extreme flatness, the following techniques may' be employed, reference being first had to the accompanying drawings wherein:

FIGURE 1 is a schematic diagram of RH processing in accordance with the instant invention.

FIGURE 2 is a schematic diagram of CH processing in accordance with the instant invention.

Essentially, two types of processing (coil and sheet) may be employed and these will be discussed separately, although it is competent to point out that the coil flattening steps of the instant invention are generally similar whether the steel is heat treated according to the RH or the CH process. Likewise, the sheet or plate flattening procedures are similar in the RH and CH processing.

Coil Flattening Procedure The coil routings, as seen in the figures of the drawings, make use of roller leveling techniques to accomplish the desired flatness. It has been generally recognized in the industry that steel must be cold worked when it is in its softest condition, i.e. after an annealing treatment,

because its hardness increases rapidly and its ductility falls off greatly as it is cold worked. This is particularly true of stainless steels which in their softest condition have a hardness at least as great as the hardness of low carbon steel after maximum cold working. Consequently, if one were to follow accepted leveling techniques, the accepted practice would be to roller level after a full anneal or, in the RH process of FIGURE 1, immediately following the l600-l800 F. austenite conditioning treatment. While standard roller leveling at such juncture will flatten annealed or annealed and austenite conditioned stainless steel, it has been found that the flatness so produced cannot be preserved through a subsequent refrigeration treatment. The dimensional instability due to temperature changes and structural changes produces distortions which leave the strip in a form no flatter than that of the austenite conditioned material.

In accordance with the instant invention, and contrary to the accepted beliefs of the industry, a roller leveling operation is employed to flatten the steel subsequent to the refrigeration step of the process. As previously explained, the steel at this stage has been transformed from the relatively soft austenitic stage to the hard martensitic condition. Yet quite surprisingly, the flexing action of a high strength roller leveler flattens the steel. Indeed, the flatness so produced is within the tolerances prescribed for one-quarter and one-half hard stainless and far better than that heretofore conceived possible in stainless steels of near maximum hardness.

The importance of this discovery will be more evident if the power requirements necessary to the level various metals are explained. Roller leveling is cold working within the plastic range. Therefore, the yield strength of the metal must be exceeded before there can be a permanent change in the shape of the material. Low carbon steels, such as those roller leveled ahead of forming into automobile body sections, for example, have a yield strength of less than 25,000 p.s.i. On the other hand, the yield strength of dead-soft precipitation hardenable stainless steel is of the order of 50,000 p.s.i., or about double that of low carbon steels. However, to show the order of magnitude of the power required for leveling the steels of this invention, the yield strength of cold worked and metallu'rgically hardened stainless steel is from 200,000 p.s.i. to 300,000 p.'s.i., or more than four times that of the same steel in the softened condition. While ordinary roller levelers are incapable of handling such stainless steels, such levelers can be successfully modifiied to exert the power necessary to flatten the hardened stainless steels.

In an illustrative embodiment of the invention utilizing RH processing techniques, the cold rolled or annealed 'coil is first subjected to an austenite conditioning treatment. This treatment is preferably accomplished in an anneal pickle line at a temperature of about 1700 F. at a speed equal to the standard annealing speed for the particular thickness of material being processed. For example, a steel strip 0.050 inch in thickness will be conditioned at 4'8 f.p.m., whereas a strip .020 in thickness will be conditioned at 60' f.p.m. After conditioning, the strip may be descaled by using an electrolytic HNO tub. Occasionally, it is found that a short dip in a HNO -HF tub is required. After descaling the strip is recoiled and transferred to the cold box for a refrigeration treatment.

At this stage in the processing, it has been found desirable to prevent the coil from being exposed to winter temperature fluctuations which would cause uncontrolled transformations and prevent the development of maximum physical properties. Preferably, the refrigeration should be accomplished as soon after the conditioning treatment as possible. However, storage may be accomplished for relatively short periods of time at temperatures above 70 F. when it is not feasible to refrigerate immediately after the conditioning step.

It is preferred to carry out the refrigeration treatment 4. in an insulated box using the expansion of liquid CO as the cooling medium. Precaution should be taken to prevent the accumulation of CO snow on the work. This may be accomplished, for example, by installing high velocity circulation fans in the box. Other methods for obtaining the necessary low temperatures, such as mechanical refrigeration, and the like, also may be used.

The refrigeration cycle consists in holding the work at a temperature of 50" F. to --75 F. for a minimum time of one to two hours, although a longer holding period may be found desirable. For example, a cycle requiring holding the work at a temperature of l00 F. for 8 hours has been successfully employed. The factors of time and temperature will vary depending upon the particular composition of the steel and the conditions of treatment; the salient point being to cold-treat the metal to achieve phase transformation and an initial increase in the metal hardness. In practice this may be accomplished by controlling the box temperature at the desired point, say -75 F., until a thermocouple inserted in the coil reaches the desired low temperature, whereupon the coil is held in the box for at least the minimum holding period.

The next step in the processing involves a flattening operation by roller leveling. It might be well to point out at this time that the strip shape (i.e. full center with tight edge or conversely, loose edge with tight center) prior to leveling determines to a large measure the success of the roller leveling operation. Most satisfactory results are obtained where a full center shape is achieved. As will be understood by the worker in the art, the leveling pressure will vary widely depending upon condition of the strip, but will be suflicient to pro duce plastic deformation to the extent required to reduce buckles and other deformations and other irregularities in the strip.

After leveling, the strip is next subjected to a final heat treatment at a temperature of about 950 F. In a preferred technique, the strip is hardened through a molten caustic-nitrate salt tub at a speed such that at least 4 minutes of exposure is obtained. The strip is then descaled in an electrolytic HNO tub; and sometimes a short immersion in an HNO HF tub is required.

It has been found that during this hardening treatment substantially all of the flatness obtained by the prior leveling operation will be retained. However, in order to assure positive flatness of the strip, the hardened material may be subjected to a second roller leveling operation following final heat treatment. This operation is optional; but as indicated, assures flatness within the ranges indicated.

After the final leveling operation, the coil may be sheared into sheets, resquared and prepared for shipment or storage.

Where the CH type of processing is to be employed, the coil is first subjected to a cold reduction of about 60%. During this rolling it has been found highly desirable to produce a slightly full center coil. Following the cold reduction the coil is subjected to the roller leveling operation which is followed by heat hardening, descaling and final flattening utilizing the techniques already described. That is, the coil, following the roller leveling operation, is subjected to a heat treatment at about 950 R, which may be conveniently accomplished by passing the strip through a molten salt tub, whereupon the strip is descaled and, if desired, subjected to a second roller leveling operation.

Sheet and Plate Flattening The flattening treatment for sheets or plates hardened in accordance with RH processing techniques must also be accomplished after the refrigeration treatment rather than after austenite conditioning. In cases where the sheets or plates are to be completely processed as individual pieces, the initial conditioning step may be performed in either a batch type of a continuous sheet HNO -HF tub, whereupon the pieces are ready for the refrigeration treatment. Insofar as the refrigeration treatment itself is concerned, the conditions described in conjunction with the coil method will be employed, the sheets or plates being stacked in the cold box and held at the required low temperature until phase transformation is completed. After refrigeration, the sheets or plates will be subjected to a roller leveling operation which, as before, will be under sutficient pressure to produce plastic deformation and the reduction of buckles and other irregularities in the sheets.

If desired, instead of completely processing the sheets as individual pieces, the stock may be handled in coil form for initial conditioning and sheared into sheets or plates either before or after the refrigeration step. If desired, an initial roller leveling operation may be conducted prior to severing the coil into individual pieces. These alternative expedients are indicated in dotted lines in FIGURE 1 of the drawing.

Basically, however, the mechanics of flattening sheets and plates are quite different from the roller leveling employed for strip material. To this end, final hardening is accomplished in a batch operation in which the sheets or plates are heated under pressure to preserve flatness. Additional improvement in flatness may be obtained by creep flattening. The hardening cycle preferably consists in heating the sheets or plates to a temperature of some 900 F. to 1000 F. for a period of at least four minutes at temperature.

In a preferred procedure, it has been found that sheet flatness can be successfully maintained by placing the sheets between steel slabs which are about 5 inches thick and machined flat, the sheets or plates being stacked one upon the other on a bottom slab, whereupon the second slab is placed over the stack. A sheet metal cover is then placed over the entire stack and a box annealing furnace lowered over the charge. The furnace temperature is raised from about 800 F. to 1150 F., preferably about 950 F., and held until the sheets or plates are soaked out for about one hour. The furnace is then removed and the charge allowed to cool with the inner cover in place. The preferred procedure for hardening between slabs is to soak out the slabs at the hardening temperature before the sheets are placed therebetween. Such preheating of the slabs is preferred in order to expedite the operation and also to minimize embrittlement of the steel.

It has also been found that in the slab type of hardening operation, it is important to have the inner cover and top slab in place until a temperature of 100 to 150 F. is reached as the charge cools, if adequate flatness is to be preserved. Otherwise, differential cooling stresses will be set-up which will warp not only the sheets but the slabs as well. As before, an optional roller leveling operation may be employed subsequent to the final heat treatment to develop maximum flatness. Similarly, the light heat tint oxide formed during the hardening operation may be removed by a brushing operation or by a light HNO -HF descaling operation, whereupon the sheets or plates may be resquared and sheared for shipment or storage.

Where the CH type of processing is to be employed, the slab hardening technique just described will be employed, the cold reduced strip being first roller leveled either before or after shearing; and similarly, following the slab hardening treatment, the sheets may be brushed and descaled.

The properties of several representative stainless steels which have been flattened in accordance with my invention are set forth in the following table, wherein steel A comprises essentially .07% carbon, 17% chromium, 7% nickel, 1% aluminum and the remainder substantially all iron, and wherein steel B comprises essentially .07% carbon, 15% chromium, 7% nickel, 2% molybdenum, 1% aluminum, and the remainder substantially all iron:

R=Retrigerated.

RH Refrigerated and hardened. C =Ooid rolled OH= Cold rolled and hardened. C0=Coil process.

S=Sheet process.

In addition to the foregoing, the various techniques described may be applied to various other types of stainless and heat resisting steels which are not precipitation hardened. For example, general purpose austenitic chromiumnickel stainless steel which can be cold worked to high tensile strengths may be readily subjected to the box hardening technique in a box annealing furnace to produce flat stress relieved sheets. Similarly, the general purpose heat hardenable corrosion and heat resisting chr0- mium steels may be subjected to roller leveling operations both before and after the hardening treatment.

Having thus described the invention in certain exemplary embodiments, what is desired to be secured and protected by Letters Patent is:

l. The method of producing extreme flatness and hardness in chromium-nickel alloy stainless steel which has been initially hardened by a phase transformation, which comprises subjecting said initially hardened steel to a final heat treatment at a temperature of from 850-l150 F. for a period of at least about four minutes and, immediately prior to said final heat treatment, subjecting said initially hardened steel to a roller leveling operation of sufficient magnitude to produce plastic deformation of said steel, said steel having a final flatness on the order of 1:005 inch under two pounds per square inch vacuum.

2. The process claimed in claim 1 wherein said phase transformation is effected by cold rolling at a reduction of about 60%.

3. The method claimed in claim 1 wherein said steel is initially hardened by means of a conditioning treatment at a temperature of about l6001800 F. followed by a sub-zero cold treatment for a period of at ileast about one to two hours.

4. The process claimed in claim 2 wherein said final heat treatment comprises a hardening treatment in a molten caustic-nitrate salt tub.

5. The method claimed in claim 2 wherein said final heat treatment is effected with said steel sheared into sheets, said treatment being conducted till. a furnace with said sheets stacked between upper and lower slabs.

6. The method claimed in claim 3 wherein [said final heat treatment is conducted by passing said steel through a molten caustic-nitrate salt bath.

7. The method claimed in claim 3 wherein said final heat treatment is conducted in a furnace with said steel sheared into sheets which are stacked between upper and lower slabs.

8. A method of producing extreme flatness in chnomium-nickel stainless steel capable of precipitation hardeningby sub-zero cold treatment followed by a heat treatment, which comprises first conditioning said steel at a temperature of from about 1600 1800 F., then subjecting said steel to a cold treatment below about 50 F. for at least about one to two hours, whereby to harden said. steel by a phase transformation, then subjecting said steel to a roller leveling operation eifective to produce plastic deformation of the steel and flatten irregularities therein, and thereafter subjecting said steel to a fin al hardening. treatment by subjecting it toheat at a temperature of from- 850 to 1150 F. for a period of time sufficien-t to obtain a substantial increase in its hardnessisaid steel having a final flatness on the order of 1.005 inch under two pounds per square inch vacuum.

9. The method claimed in claim 8 wherein said final hardening treatment is conducted in a box anneal furnace, the steel having been cut into sheets which are stacked between upper and lower slabs which have been previously pre-heated' and soaked out at temperature.

10. The method claimed in claim 9 wherein said final heat hardening treatment is conducted with a cover over saidslabs and the sheets therebetween, and wherein, subsequent to said heat treatment, said sheets are allowed to cool with said cover in place until a temperature of from 100 F. to 150 F. is reached.

11. A method of producing extreme flatness in chromium-nickel stainless steel capable of precipitation hardeniing, which includes :the step of transforming said steel from the austenite to martensite condition, thereafter subjecting said steel to a roller leveling operation effective to produce plastic deformation-wot the steel and flatten irregularities therein, and thereafter subjecting said steel to a final hardening treatment by subjecting it to heat at 'a temperature of from 850 to 1150 F. in a box anneal furnace, said steel having been cut into sheets which are stacked between upper and lower slabs which have been previously pre-heated and soaked out at temperature, including the step of placing a cover over said stacked sheets and slabs during said final heat treatment, and, subsequent to said final heat treatment, permitting said sheets to cool with the said cover in place until a tempenature of from 100 F. to 150 F. is reached, said steel having a finail flatness on the order of 1.005 inch under two pounds per square inch vacuum.

References Cited in the file of this patent UNITED STATES PATENTS 1,929,356 Janitzky Oct. 3, 1933 1,982,810 Hickman Dec. 4, 1934 2,358,799 Franks Sept. 26, 1944 2,799,602 Lena July 1 6, 1957 2,890,143 Bingel June 9, 1959 2,969,299 Fullerton et al. Jan; 24, 1961 FOREIGN PATENTS 446,676 Canada Feb. 1 0, 1948 OTHER REFERENCES MetalsHandbo-ok, 1948 Edition, page 628, The Making, Shaping and Treating of Steel, Seventh Edition, United States Steel, 1957, page 863. 

1. THE METHOD OF PRODUCING EXTERME FLATNESS AND HARDNESS IN CHROMIUM-NICKEL ALLOY STAINLESS STEEL WHICH HAS BEEN INITIALLY HARDENED BY A PHASE TRANSFORMATION, WHICH COMPRISES SUBJECTING SAID INITIALLY HARDENED STEEL TO A FINAL HEAT TREATMENT AT A TEMPERATURE OF FROM 850*-1150* F. FOR A PERIOD OF AT LEAST ABOUT FOUR MINUTES AND, IMMEDIATELY PRIOR TO SAID FINAL HEAT TREATMENT, SUBJECTING SAID INITIALLY HARDENED STEEL TO A ROLLER LEAVING OPERATION OF SUFFICIENT MAGNITUDE TO PRODUCE PLASTIC DEFORMATION OF SAID STEEL, SAID STEEL HAVING A FINAL FLATNESS ON THE ORDER OF$.005 INCH UNDER TWO POUNDS PER SQUARE INCH VACUUM. 